Cyanide is a highly toxic compound that is readily available in scientific laboratories and industrial settings. Moreover, it is relatively easy to synthesize from inexpensive, widely obtainable reagents. Thus, cyanide has the potential to come into the hands of terrorists, and to be used as a weapon of mass destruction. Cyanide would be particularly lethal when released as a gas in closed spaces such as airports or train stations. Only two cyanide antidotes are available in the United States -- sodium thiosulfate, and nitrites in the form of sodium nitrite and amyl nitrite. Both agents have significant side effects and serious limitations, including they would be impractical in treating a large number of unconscious persons from mass cyanide exposure, because sodium thiosulfate and sodium nitrite must be administered intravenously, and amyl nitrite must be inhaled for several minutes. Cobinamide, the penultimate compound in the biosynthesis of cobalamin (vitamin B12), binds two cyanide molecules with extremely high affinity (~1022 M-1). We have shown that cobinamide is a remarkably effective cyanide antidote in mice, cultured mammalian cells, and Drosophila melanogaster. Mice can be rescued from a cyanide dose of two times the LD50 with an intramuscular injection of cobinamide, making cobinamide an attractive agent for treating mass casualties. We now propose to perform the requisite pre-clinical studies of cobinamide to obtain Investigator's New Drug (IND) approval from the Food and Drug Administration (FDA). The pre-clinical studies will be completed during the first three grant years, and during grant years four and five, we propose to perform Phase I and lla clinical studies. The Phase II studies will be performed on acutely hypertensive patients treated with nitroprusside;cobinamide could allow nitroprusside to be given for longer periods and at higher doses because nitroprusside therapy is limited by cyanide toxicity. Cobinamide would be a welcome addition to drug therapies directed against cyanide exposure;in addition to countering a terrorist attack and reducing nitroprusside toxicity, cobinamide could be used to treat smoke inhalation victims, occupational and industrial cyanide exposures, cigarette smokers, and hemodialysis patients.